There is conventionally an imaging method of irradiating an object with radiation and generating an image on the basis of the intensity of the radiation that has passed through the object. In this method, when imaging is executed using an analog film, the dose of X-rays is adjusted such that the observation region has a predetermined density. To the contrary, in digital imaging in which data obtained by a kind of imaging apparatus such as a sensor or camera is displayed on a monitor screen or X-ray diagnostic film, the image density after imaging can arbitrarily-be adjusted by image processing. For this reason, digital data is sometimes acquired by irradiating an object with X-rays in minimum dose, and the gray level of the obtained data is converted to easily observe it (e.g., Japanese Patent Laid-Open No. 2002-245453). In this case, image processing is executed such that the observation region has a predetermined density. An amount that is acquired from digital data and is to be used for the processing is called a feature amount. In, e.g., frontal chest imaging, it is demanded that the fifth intercostal density should be 1.8 to 2.0. The fifth intercostal region is extracted analytically (on the basis of various algorithms) from digital data. A statistical amount (e.g., the average or mode) is calculated from the digital data in that region to obtain a feature amount. Gray level conversion is executed by image processing such that the feature amount (digital value) represents a predetermined density. That is, the feature amount indicates the representative value of the digital data in the observation region or a value that highly correlates to the digital data. To calculate such a feature amount, for example, the two-dimensional structure of an entire object is analyzed to extract a predetermined region, and a feature amount is calculated in the predetermined region. Alternatively, a feature amount is calculated by histogram analysis.
However, the analytic function to be used to calculate the above-described feature amount must be prepared for each part of an object, resulting in high development cost. In addition, a portion related to an undeveloped analytic function cannot be imaged. Furthermore, the parameters of a gray level conversion curve must be defined by empirical rules. Conventionally, the contrast is adjusted by trial and error while visually confirming it. There are no objective indicators representing the suitability of the contrast.